E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
says Geoffrey of Monmouth, was erected by Merlin (the magician) to perpetuate the treachery of Hengist, who desired a friendly meeting with Vortigern, but fell upon him and his 400 attendants, putting them all to the sword. Aurelius Ambrosius asked Merlin to recommend a sensible memento of this event, and Merlin told the king to transplant the Giants Dance from the mountain of Killaraus, in Ireland. These stones had been brought by the giants from Africa as baths, and all possessed medicinal qualities. Merlin transplanted them by magic. This tale owes its birth to the word stan-hengist, which means uplifted stones, but hengist suggested the name of the traditional hero.
Stonehenge, once thought a temple, you have found
A throne where kings, our earthly gods, were crowned,